The ECU has two sides. One is for talking to things outside the vehicle - OBD-II or in our case Digital Wrench. It is used by scan tools (diagnostic thingies) to see and tweak what is going on in a generic way.
The other side of the ECU talks to all the other computer thingies in the vehicle. That is CAN Bus. Speed sensors, ABS, stability control, fuel control, blah, all talk to the ECU on the CAN bus.
Given the 10s of millions of vehicles produced each year, micro-controllers (little computers) that speak CAN are abundant and affordable.
This computer CPU talks CAN, and is $7.20:
MKE18F512VLL16 NXP / Freescale | Mouser
It also speaks I2C, SPI, UART, so connecting to an Arduino or Raspberry Pi should be cakewalk.
Tons of other choices.
Sniffing the data should be very easy. Decoding what 0x1a 0x23 0x11 0xb6 0xff 0x23 means, is likely to be quite a chore. Though, if you could id the parts you want to talk to, a data sheet downloaded for free might be a big step forward.
Fun idea, worth looking into further. Ya' ain't gonna be able to update the tune/ECU that way, though.